The present invention relates to an exhaust gas purifying device for an engine, and, more specifically, to an exhaust gas purifying device for an engine to remove NOx from exhaust gas of the engine.
An exhaust gas purifying device for an engine that performs a lean burn operation is known. Such a device is equipped with a NOx trap catalyst including a NOx trap material to absorb NOx during a state of high oxygen concentration in the exhaust gas and release the absorbed NOx during a state of low oxygen concentration in the exhaust gas. The NOx absorption capacity of the NOx tarp material is limited. Accordingly, in this exhaust gas purifying device, when the amount of NOx absorbed in the NOx tarp material reaches a saturation amount, a purifying control (so called, rich-spike control) is performed which changes an engine operating condition from the lean burn operation to an operation with a stoichiometric air-fuel ratio or the like to lower the concentration of oxygen in the exhaust gas. This control causes the NOx trap material to release NOx absorbed and to reduce and thereby purify the released NOx with a catalytic metal included in the NOx trap catalyst.
As a device for conducting a diagnosis of deterioration for this kind of NOx trap catalyst, Japanese Patent Laid-Open Publication No. 2000-337131 proposes a device making use of a peak value of an output of the NOx sensor during the rich-spike control. In this device, the deterioration is determined such that the lower the peak value of the NOx sensor at a transitional stage to a rich side becomes, the more NOx trap capacity of the NOx trap material has reduced, that is, the NOx trap catalyst has deteriorated.
The NOx trap catalyst, in general, includes a catalytic metal (noble metal) for oxidizing NOx and an O2 storage material for absorbing oxygen. However, the deterioration diagnosis device described in the above patent publication may not diagnose the deterioration of the reduction capacity of the catalytic metals or deterioration of O2 storage capacity of the O2 storage material.
Further, when the rich-spike control is executed, the following reactions occur sequentially: O2 release from the O2 storage material; oxidization of an exhaust gas composition by the released O2; NOx release from the NOx trap material; and reduction of the released NOx by the catalytic metal. However, if the air-fuel ratio of the exhaust gas is changed rapidly toward a rich side during NOx release, as performed in the deterioration diagnosis device described in the above publication, results of some reactions described above adversely affect or are mixed up with the output value of the NOx sensor due to an output delay of the NOx sensor or the like. Accordingly, a state of these reactions is not detected appropriately, so that each of deteriorations of O2 storage capacity of the O2 storage material (deterioration of O2 storage material) and reduction capacity of the catalytic metal (deterioration of catalytic metal) can not be diagnosed accurately.
The present invention has been devised in view of the above-described problems, and an object of the present invention is to provide an exhaust gas purifying device for an engine that performs the diagnosis function of diagnosing accurately each of deterioration of O2 storage capacity of an O2 storage material and NOx reduction capacity of a catalytic metal, which are included in a NOx trap catalyst.
In order to achieve the above-described object, the first aspect of the present invention provides an exhaust gas purifying device for an engine, comprising a NOx trap catalyst disposed in an exhaust gas passage of the engine including a NOx trap material to absorb NOx while oxygen of an exhaust gas is in high concentration and release the absorbed NOx while oxygen is in low concentration, a catalytic metal to purify the NOx released from the NOx trap material, and an O2 storage material to absorb O2 while oxygen of the exhaust gas is in high concentration and release the absorbed O2 while oxygen is in low concentration, a NOx sensor disposed in the exhaust gas passage downstream of the NOx trap catalyst, NOx absorbed amount calculating section for calculating the amount of NOx absorbed in the NOx trap catalyst, NOx release controlling section for lowering the concentration of oxygen of the exhaust gas in the exhaust gas passage so as to cause the NOx trap catalyst to release NOx absorbed when the amount of the NOx absorbed in the NOx trap catalyst reaches a specified amount, operating condition detecting section for detecting an operating condition of the engine, determination section for determining whether a specified diagnosis condition to diagnose the NOx trap catalyst is satisfied or not, according to the engine operating condition detected by the operating condition detecting section, detecting section for detecting whether or not O2 release from the O2 storage material caused by oxygen concentration lowering of the exhaust gas in the exhaust gas passage by the NOx release controlling section has terminated, diagnosing section for diagnosing a degree of deterioration of respective capacities of the O2 storage material and the catalytic metal that are included in the NOx trap catalyst when the specified diagnosis condition is satisfied, the diagnosing section diagnosing a degree of deterioration of O2 storage capacity of the O2 storage material according to output values of the NOx sensor that are generated during a term from the beginning of the oxygen concentration lowering of the NOx release controlling section until a termination of the O2 release from the O2 storage material, the diagnosing section diagnosing a degree of deterioration of NOx reduction capacity of the catalytic metal according to an output value of the NOx sensor that is generated after the termination of the O2 release from the O2 storage material, wherein the NOx release controlling section is configured such that a speed of lowering oxygen concentration thereby when the specified diagnosis condition is satisfied is slower than that when the specified diagnosis condition is not satisfied.
According to the exhaust gas purifying device for an engine of the first aspect of the invention, each of deterioration of O2 storage capacity of the O2 storage material and NOx reduction capacity of the catalytic metal, which are included in the NOx trap catalyst, can be diagnosed accurately, during a control (rich-spike control) for NOx release from the NOx trap catalyst for lowering the concentration of oxygen of the exhaust gas in the exhaust gas passage.
The second aspect of the present invention provides the exhaust gas purifying device of an engine of the first aspect of the invention, wherein the detecting section comprise an O2 sensor disposed in the exhaust gas passage downstream of the NOx trap catalyst, and O2 release termination determining section for determining that the O2 release from the O2 storage material has terminated when an output value of the O2 sensor indicates a value equivalent to a stoichiometric air-fuel ratio of the exhaust gas.
The third aspect of the present invention provides the exhaust gas purifying device of an engine of the first aspect of the invention, wherein the diagnosing section is configured so as to diagnose that O2 storage capacity of the O2 storage material included in the NOx trap catalyst has deteriorated when an integrated value of output values of the NOx sensor with respect to time during a term from the beginning of the oxygen concentration lowering of the NOx release controlling section until a termination of the O2 release from the O2 storage material is smaller than a first threshold value.
The fourth aspect of the present invention provides the exhaust gas purifying device of an engine of the first aspect of the invention, wherein the diagnosing section is configured so as to diagnose that O2 storage capacity of the O2 storage material included in the NOx trap catalyst has deteriorated when a value that is gained by dividing a difference between a maximum value and a minimum value of output values of the NOx sensor that are generated during a term from the beginning of the oxygen concentration lowering of the NOx release controlling section until a termination of the O2 release from the O2 storage material, by the time spent during the term is greater than a second threshold value.
According to the exhaust gas purifying device for an engine of the third and fourth aspects of the invention, the deterioration of O2 storage capacity of the O2 storage material included in the NOx trap catalyst can be detected by making use of the output values of the NOx sensor that are generated during the term from the beginning of rich-spike control by the NOx release controlling section until the termination of the O2 release from the O2 storage material.
The fifth aspect of the present invention provides the exhaust gas purifying device of an engine of the third aspect of the invention, further comprising first correcting section for correcting the first threshold value, in such a way that the greater a difference between the amount of absorbed NOx calculated by the NOx absorbed amount calculating section and the specified amount is, the greater value the first threshold value for diagnosing the deterioration of O2 storage capacity of the O2 storage material included in the NOx trap catalyst is corrected to.
The sixth aspect of the present invention provides the exhaust gas purifying device of an engine of the fourth aspect of the invention, further comprising second correcting section for correcting the second threshold value, in such a way that the greater a difference between the amount of absorbed NOx calculated by the NOx absorbed amount calculating section and the specified amount is, the smaller value the second threshold value for diagnosing the deterioration of O2 storage capacity of the O2 storage material included in the NOx trap catalyst is corrected to.
When the extremely larger amount of NOx than the specified amount has been absorbed for some reason, such as under a situation that the engine has been shut down during the previous rich-spike control, executing the deterioration diagnosis of the O2 storage material by using a normal threshold value may provide an erroneous diagnosis that the O2 storage material has not deteriorated yet, in spite of the fact that it has already deteriorated, because of too much NOx absorbed in the NOx trap catalyst. According to the exhaust gas purifying device for an engine of these aspects of the invention, however, when it is determined that the amount of NOx absorbed in the NOx trap catalyst is greater than the specified amount for determining the beginning of rich-spike control, the threshold value for determining the deterioration of O2 storage capacity is corrected in such a manner that determination of the deterioration of O2 storage capacity becomes apt to occur, according to the difference between the NOx absorbed amount and the specified amount for the determination. Accordingly, the above-described erroneous diagnosis can be prevented.
The seventh aspect of the present invention provides the exhaust gas purifying device of an engine of the first aspect of the invention, wherein the diagnosing section is configured so as to diagnose that NOx reduction capacity of the catalytic metal included in the NOx trap catalyst has deteriorated when a maximum value of output values of the NOx sensor that are generated after a termination of the O2 release from the O2 storage material is greater than a third threshold value.
The eighth aspect of the present invention provides the exhaust gas purifying device of an engine of the first aspect of the invention, wherein the diagnosing section is configured so as to diagnose that NOx reduction capacity of the catalytic metal included in the NOx trap catalyst has deteriorated when an integrated value of output values of the NOx sensor with respect to time during a term from a termination of the O2 release from the O2 storage material until a specified time has passed since the termination of the O2 release is greater than a fourth threshold value.
The ninth aspect of the present invention provides the exhaust gas purifying device of an engine of the first aspect of the invention, wherein the diagnosing section is configured so as to diagnose that NOx reduction capacity of the catalytic metal included in the NOx trap catalyst has deteriorated when a value that is gained by dividing a difference between a maximum value output values of the NOx sensor that are generated during a term from a termination of the O2 release from the O2 storage material until a specified time has passed since the termination of the O2 release and an output value of the NOx sensor that is generated at the time when the O2 release from the O2 storage material has terminated, by a difference between the output value of the NOx sensor that is generated at the time when the O2 release from the O2 storage material has terminated and an output value of the NOx sensor that is generated at the time when the specified time has passed is greater than a fifth threshold value.
According to the exhaust gas purifying device for an engine of the seventh through ninth aspects of the invention, the deterioration of NOx reduction capacity of the catalytic metal included in the NOx trap catalyst can be detected by making use of the output values of the NOx sensor that are generated during a specified term after the termination of the O2 release from the O2 storage material.
The tenth aspect of the present invention provides the exhaust gas purifying device of an engine of the eighth aspect of the invention, wherein the diagnosing section further comprising second correcting section for correcting the fourth threshold value, in such a way that the greater a difference between the amount of absorbed NOx calculated by the NOx absorbed amount calculating section and the specified amount is, the greater value the fourth threshold value is corrected to.
When the extremely larger amount of NOx than the specified amount has been absorbed for some reason, such as under a situation that the engine has been shut down during the previous rich-spike control, executing the deterioration diagnosis of the catalytic metal by using a normal threshold value may provide an erroneous diagnosis that the catalytic metal has not deteriorated yet, in spite of the fact that it has already deteriorated, because of too much NOx absorbed in the NOx trap catalyst. According to the exhaust gas purifying device for an engine of the tenth aspect of the invention, however, when it is determined that the amount of NOx absorbed in the NOx trap catalyst is greater than the specified amount for determining the beginning of rich-spike control, the threshold value for determining the deterioration of catalytic metal capacity is corrected in such a manner that determination of the deterioration of catalytic metal capacity becomes unlikely to occur, according to the difference between the NOx absorbed amount and the specified amount for the determination. Accordingly, the above-described erroneous diagnosis can be prevented.
The eleventh aspect of the present invention provides the exhaust gas purifying device of an engine of the first aspect of the invention, further comprising NOx trap capacity diagnosing section for diagnosing that NOx trap capacity of the NOx trap material included in the NOx trap catalyst has deteriorated when the sum of an integrated value of output values of the NOx sensor with respect to time during a term from the beginning of the oxygen concentration lowering of the NOx release controlling section until a termination of the O2 release from the O2 storage material and an integrated value of output values of the NOx sensor with respect to time during a term from the termination of the O2 release from the O2 storage material until a specified time has passed since the termination of the O2 release is smaller than a sixth threshold value.
The twelfth aspect of the present invention provides the exhaust gas purifying device of an engine of the first aspect of the invention, further comprising specified-amount correcting section for correcting the specified amount of NOx absorbed amount that is used by the NOx release controlling section in executing the NOx release to a smaller value when it is diagnosed that NOx reduction capacity of the catalytic metal included in the NOx trap catalyst has deteriorated.
According to the exhaust gas purifying device for an engine of the twelfth aspect of the invention, when NOx reduction capacity of the catalytic metal included in the NOx trap catalyst has deteriorated, that is, when the amount of NOx which can be reduced by the catalytic metal, the rich-spike control is performed in the condition where the amount of NOx absorbed in the NOx trap catalyst is small. Accordingly, this can decrease the amount of NOx released during the rich-spike control and the amount of NOx to be reduced by the catalytic metal, resulting in lightening NOx reduction capacity required in the catalytic metal.
The thirteenth aspect of the present invention provides the exhaust gas purifying device of an engine of the first aspect of the invention, further comprising oxygen concentration correcting section for setting the concentration of oxygen of the exhaust gas in the exhaust gas passage, which is lowered by the NOx release controlling section, to a higher value when it is diagnosed that O2 storage capacity of the O2 storage material included in the NOx trap catalyst has deteriorated.
The concentration of oxygen of the exhaust gas in the exhaust gas passage during the rich-spike control is set to a low value in expectation of O2 release from the O2 storage material. Accordingly, if O2 storage capacity of the O2 storage material has been decreased, an expected amount of O2 may not be released, resulting in a state of oxygen deficit. Herein, according to the exhaust gas purifying device for an engine of the thirteenth aspect of the invention, when O2 release capacity of the O2 storage material included in the NOx trap catalyst has deteriorated, that is, when the mount of O2 released from the O2 storage material during the rich-spike control decreases, for example, the oxygen concentration of the exhaust gas in the exhaust gas passage during the rich-spike control is set to a high value by an air-fuel control or the like. Accordingly, the shortage of O2 released from the O2 storage material can be compensated.
The fourteenth aspect of the present invention provides the exhaust gas purifying device of an engine of the first aspect of the invention, further comprising second diagnosing section for diagnosing that O2 storage capacity of the O2 storage material has deteriorated when the time spent from the beginning of the oxygen concentration lowering of the NOx release controlling section until a termination of the O2 release from the O2 storage material is shorter than a specified time.
According to the exhaust gas purifying device for an engine of the fourteenth aspect of the invention, the diagnosis of deterioration of the O2 storage material is executed based on only the time of O2 release from the O2 storage material.
The fifteenth aspect of the present invention provides the exhaust gas purifying device of an engine of the fourteenth aspect of the invention, wherein the second diagnosing section further comprising third correcting section for correcting the specified time to be used for diagnosing O2 storage capacity of the NOx trap catalyst, in such a way that the greater a difference between the amount of absorbed NOx calculated by the NOx absorbed amount calculating section and the specified amount is, the longer value the specified time is corrected to.
The sixteenth aspect of the present invention provides an exhaust gas purifying device of an engine, comprising, a NOx trap catalyst disposed in an exhaust gas passage of the engine including a NOx trap material to absorb NOx while oxygen of an exhaust gas is in high concentration and release the absorbed NOx while oxygen is in low concentration, a catalytic metal to purify the NOx released from the NOx trap material, and an O2 storage material to absorb O2 while oxygen of the exhaust gas is in high concentration and release the absorbed O2 while oxygen is in low concentration, a NOx sensor disposed in the exhaust gas passage downstream of the NOx trap catalyst, an O2 sensor disposed in the exhaust gas passage downstream of the NOx trap catalyst, an engine speed sensor operative to detect an engine speed, an accelerator opening sensor operative to detect an opening of an accelerator, an engine coolant temperature sensor operative to detect a temperature of an engine coolant, a control unit operative to receive output values of the respective sensors, and to control a fuel injector and an ignition timing of the engine and execute a diagnosis of deterioration of the NOx trap catalyst according to the received output values of the sensors, wherein the control unit calculates the amount of NOx absorbed in the NOx trap catalyst according to an output value of the NOx sensor, controls the fuel injector so as to lower the concentration of oxygen of the exhaust gas in the exhaust gas passage and thereby to cause the NOx trap catalyst to release NOx absorbed when the amount of the NOx absorbed in the NOx trap catalyst reaches a specified amount, determines that a specified diagnosis condition to diagnose the NOx trap catalyst is satisfied when the engine is warmed up with the engine coolant temperature that is greater than a specified temperature and the engine is in an ordinary state with a changing rate of the accelerator opening that is lower than a specified rate, detects whether or not O2 release from the O2 storage material caused by oxygen concentration lowering of the exhaust gas in the exhaust gas passage has terminated according to the output value of the O2 sensor, diagnoses, when the specified diagnosis condition is satisfied, that O2 storage capacity of the O2 storage material included in the NOx trap catalyst has deteriorated according to output values of the NOx sensor that are generated during a term from the beginning of the oxygen concentration lowering until a termination of the O2 release from the O2 storage material and that NOx reduction capacity of the catalytic metal included in the NOx trap catalyst has deteriorated according to an output value of the NOx sensor that is generated after a termination of the O2 release from the O2 storage material, and controls the fuel injector such that a speed of lowering oxygen concentration when the specified diagnosis condition is satisfied is slower than that when the specified diagnosis condition is not satisfied.
The seventeenth aspect of the present invention provides an exhaust gas purifying device of an engine, comprising a NOx trap catalyst disposed in an exhaust gas passage of the engine including a NOx trap material to absorb NOx while oxygen of an exhaust gas is in high concentration and release the absorbed NOx while oxygen is in low concentration, a catalytic metal to purify the NOx released from the NOx trap material, and an O2 storage material to absorb O2 while oxygen of the exhaust gas is in high concentration and release the absorbed O2 while oxygen is in low concentration, a NOx sensor disposed in the exhaust gas passage downstream of the NOx trap catalyst, an O2 sensor disposed in the exhaust gas passage downstream of the NOx trap catalyst, an engine speed sensor operative to detect an engine speed, an accelerator opening sensor operative to detect an opening of an accelerator, an engine coolant temperature sensor operative to detect a temperature of an engine coolant, a control unit operative to receive output values of the respective sensors, and to control a fuel injector and an ignition timing of the engine and execute a diagnosis of deterioration of the NOx trap catalyst, according to the received output values of the sensors, wherein the control unit calculates the amount of NOx absorbed in the NOx trap catalyst according to an output value of the NOx sensor, controls the fuel injector so as to lower the concentration of oxygen of the exhaust gas in the exhaust gas passage and thereby to cause the NOx trap catalyst to release NOx absorbed when the amount of the NOx absorbed in the NOx trap catalyst reaches a specified amount, determines that a specified diagnosis condition to diagnose the NOx trap catalyst is satisfied when the engine is warmed up with the engine coolant temperature that is greater than a specified temperature and the engine is in an ordinary state with a changing rate of the accelerator opening that is lower than a specified rate, detects whether or not O2 release from the O2 storage material caused by oxygen concentration lowering of the exhaust gas in the exhaust gas passage has terminated according to the output value of the O2 sensor, diagnoses, when the specified diagnosis condition is satisfied, that O2 storage capacity of the O2 storage material included in the NOx trap catalyst has deteriorated when an integrated value of output values of the NOx sensor with respect to time during a term from the beginning of the oxygen concentration lowering until a termination of the O2 release from the O2 storage material is smaller than a first threshold value and that NOx reduction capacity of the catalytic metal included in the NOx trap catalyst has deteriorated when a maximum value of output values of the NOx sensor that are generated after a termination of the O2 release from the O2 storage material is greater than a second threshold value, and controls the fuel injector such that a speed of lowering oxygen concentration when the specified diagnosis condition is satisfied is slower than that when the specified diagnosis condition is not satisfied.